Our work with laboratory animals relates the variation in susceptibility to spontaneous site-specific tumors to the variation in dietary practices. Such information may provide the rationale for manipulating the diet in order to reduce the risk of certain spontaneous neoplasms. The ongoing work is based on the following observations: when non-inbred rats are given freedom of dietary choice, the quantity and composition of the diet selected varies from animal to animal in a way that greatly increases the risk to the individual of developing a number of site-specific epithelial tumors. Two hypotheses are proposed: (1) that there is individual specificity in the dietary conditions that moderate tumor susceptibility. This hypothesis is being tested by determining whether or not susceptibility is reduced when an animal is maintained on a physiologically adequate diet that differs from the one of its own choosing. (2) that multiple interacting temporal-specific dietary and dietary-related factors early in life moderate tumor susceptibility throughout an individual's life-time. These conditions hav been expressed in the form of a mathematical model. The model and thereby the hypothesis is being tested by determining whether the likelihood that an individual will develop a tumor can be estimated on the basis of its dietary practices and growth responses and by determining the extent to which the occurrence of spontaneous tumors of the rat can be reduced when the dietary conditions that contribute to risk are "minimized" and those that lower risk are "maximized" at the appropriate ages.